The 3rd generation partnership project (3GPP) specification for global system for mobile communication (GSM) includes the specifications for GPRS, EDGE, and W-CDMA. Current working assumption for the physical layer multiple access scheme is OFDMA for the downlink, and single carrier FDMA (SC-FDMA) for the uplink. The subcarrier spacing in the OFDM downlink is 15 kHz and there are a maximum of 2048 subcarriers available. Supported modulation formats on the downlink data channels are QPSK, 16QAM, and 64QAM.
According to the 3GPP long term evolution (LTE), the transmission is divided in time into time slots of duration 0.5 ms, and a subframe of duration of 1.0 ms. A radio frame is 10 ms long. The basic transmission unit is a transmit time interval (TTI). A TTI can be one or more time slots.
During each time slot, multiple OFDM symbols are transmitted. Each time slot can include pilot tones and user data. The pilot tones enable channel estimation at a receiver for data demodulation, antenna selection, and radio resource allocation. Typically, the receiver sends back the channel estimates to the transmitter, and the transmitter can then select antennas according to the best channels. In a dynamic environment, channel estimation and antenna selection is performed periodically to accommodate varying channel conditions and mobile stations.
Each OFDM symbol starts with a cyclic prefix (CP). The CP minimizes inter-symbol interference (ISI) in the presence of multipath interference in a wireless channel. When the first CP-length duration of the received signal is discarded, the resultant signal turns out to be a circular convolution of the data with the channel. This is true as long as the multipath dispersion in the channel is less than the CP duration. Therefore, interference between the subcarriers, i.e., inter-carrier interference (ICI), does not exist.
OFDMA Structure in LTE
The basic uplink transmission scheme is described in 3GPP TR 25.814, v1.2.2 “Physical Layer Aspects for Evolved UTRA,” Release 7, March 2006, incorporated herein by reference. That scheme is a single-carrier transmission (SC-FDMA) with cyclic prefix to achieve uplink inter-user orthogonally to enable efficient frequency-domain equalization at the receiver. This allows for a relatively high degree of commonality with the downlink OFDM scheme, and the same parameters, e.g., clock frequency, can be reused.
Antenna Selection
The 3GPP systems use multiple-input, multiple-output communications (MIMO). That is, the transmitters and receivers can use an array of antennas to increase spatial. While MIMO systems perform well, they also increase the hardware and signal processing complexity, power consumption, and component size in the transmitter and the receiver. One of the main reasons for the increase in complexity is that each receive antenna element requires a dedicated radio frequency (RF) chain that comprises a low noise amplifier, a frequency down-converter, and an analog to digital converter; and each transmit antenna element requires an RF chain that comprises a digital to analog converter, a frequency up-converter, and a power amplifier. Moreover, processing the signals received in spatial multiplexing schemes or with space-time trellis codes requires sophisticated receivers whose complexity increases, sometimes exponentially, with the number of transmit and receive antenna elements.
Antenna selection addresses some of the complexity drawbacks associated with MIMO systems because antenna elements are typically cheap, and the RF chains are considerably more expensive. Antenna selection reduces the hardware complexity of transmitters and receivers by using fewer RF chains than the number of antenna elements. In antenna selection, a subset of the available antenna elements is adaptively selected by a switch, and only signals from the selected subset of antennas are processed further by the available RF chains, N. B. Mehta and A. F. Molisch, “Antenna selection in MIMO systems,” in “MIMO System Technology for Wireless Communications,” (G. Tsulos, ed.), ch. 6, CRC Press, 2006, A. F. Molisch and M. Z. Win, “MIMO systems with antenna selection,” IEEE Microwave Mag., pp. 46-56, March 2004, S. Sanayei and A. Nosratinia, “Antenna selection in MIMO systems,” IEEE Commun. Mag., pp. 68-73, October 2004, R1-050944, “Antenna selection techniques,” Mitsubishi Electric, 3GPP RANI#42, 2005, R1-051398, “Transmit antenna selection techniques for uplink E-UTRA,” Institute for Infocomm Research, Mitsubishi Electric, NTT DoCoMo, all incorporated herein by reference.
Being able to switch antennas within a TTI could be advantageous because this would enable transmission of pilot tones or data from many or all of the available antennas to increase spatial diversity. That is, signals transmitted from different antennas reach a receiver through different wireless channels at any time. This improves the robustness of transmission schemes to fading since the odds of all the wireless channels being in a fading dip are considerably lower.